Bringing Peccei-Quinn Mechanism Down to Earth

TL;DR

This paper explores the possibility that the Peccei-Quinn mechanism and associated axion could operate at energies below the weak scale, offering new insights into naturalness, Higgs mass, and potential experimental signatures.

Contribution

It proposes a model where the PQ boson and axion are light and accessible at low energies, providing a novel approach to naturalness and potential explanations for experimental anomalies.

Findings

The model can explain the Belle II excess as missing energy from light PQ bosons.

The scenario maintains finite naturalness against quantum corrections.

Potential to distinguish PQ boson and axion contributions in meson decays.

Abstract

It is conventionally assumed that the physics underlying the Peccei-Quinn (PQ) mechanism for addressing the strong CP problem is at very high energies, orders of magnitude above the weak scale. However, this may not be the case in general and the associated PQ boson $\phi$, besides the signature state, {\it i.e.} the ultralight axion $a$, may emerge well below the weak scale. We consider this possibility and examine some of the conditions for its viability. The example model proposed here may also provide the requisite Standard Model Higgs mass parameter, without invoking new scalars above the GeV scale. The corresponding parameter space can maintain {\it finite naturalness} against quantum corrections. This scenario, depending on choice of parameters, can potentially be constrained by flavor data. We point out that the current mild excess in $B^+\to K^+ \nu \bar \nu$, reported by the Belle II experiment, could be explained in this setup as $B^+\to K^+ \phi$ and $B^+\to K^+ a$, with both $\phi$ and $a$ escaping the detector as missing energy. For a sufficiently heavy PQ boson, in the GeV regime, one can separate these two contributions, due to the difference in $K^+$ momenta. In this case, the axion may also affect lighter meson, {\it e.g.} kaon, decays while $\phi$ would not be a kinematically allowed final state.